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Answer To Insect Question From A Gardener

January 23, 2001|By ERIC T. NATWICK

University of California—Imperial

County Cooperative Extension

Question: I've noticed some small flies, what I would call gnats, in the soil of my houseplants. This winter I've also seen them all over the top, new growth of my tomato plants in the garden. Both the houseplants and tomatoes seem to be surviving and thriving. I sprayed the tomatoes with Safer Soap insecticide and that reduced the numbers some. Should I be concerned about these "flies?"

Although I did not view the pests for a positive identification, it is likely fungus gnats (families Mycetophilidae and Sciaridae) were the "flies" in the houseplant soil. Fungus gnats occur around damp, decaying vegetation, algae and fungi. These flies can appear in large numbers in or around buildings, prompting complaints. Fungus gnats infest soil, houseplants and container media, where larvae feed on organic matter and roots.

Adult fungus gnats are primarily a nuisance. Fungus gnats can both enter buildings as flying adults and develop indoors through all life stages. Fungus gnats do not bite people or animals and in the United States are not known to carry human pathogens.


Fungus gnat larvae damage indoor plants by feeding on roots, stunting plant growth. Root damage occurs in houseplants if high populations infest frequently watered, organic-rich soil. Fungus gnat larval damage can be especially serious in greenhouses and nurseries. In addition to larvae chewing on roots, both larvae and adults can spread plant pathogens and may promote plant disease.

Adult fungus gnats are dark, delicate insects, resembling mosquitoes. Adults have slender legs with long, segmented antennae longer than the head. Adults of most species are about 1/16 to 1/8 inch long.

Fungus gnats wings are light gray to clear but are relatively weak fliers; they often run or rest on growing media, foliage or litter. Females lay tiny eggs in moist organic debris or potting soil. Larvae have a shiny black head and an elongated, whitish to clear body. They eat organic mulch, leaf mold, compost, root hairs and fungi. Fungus gnats have several generations per year. Adults feed little, consuming only liquids such as water or flower nectar. The tiny eggs and oblong pupae occur in damp places where larvae feed.

To determine whether there is a fungus gnat problem, visually inspect for adults that may be resting on plants, soil, windows or walls. They may be seen flying. Besides looking for adults, check outside near buildings for excessively moist conditions and organic debris; larvae may be feeding there and these are places to take control actions as discussed below.

Manage adult fungus gnats by screening windows and doors, reducing moisture and organic debris. Biological control agents are available to control fungus gnats. Insecticides are used in commercial plant production but are not generally recommended for control around the home. Most of these insects' lifespan is spent as larvae and pupae in organic matter or soil, so most control methods target the immature stages, not the mobile and short-lived adults.

Fungus gnats thrive on decaying vegetation and fungi under moist conditions; avoid excessive watering and provide good drainage. Clean up free-standing water and allow container soil to dry out between watering. Do not use incompletely composted organic matter in potting media unless it is pasteurized first because it often is infested with fungus gnats. Moist and decomposing grass clippings, compost, organic fertilizers and mulches are a favorite breeding spot. Minimize organic debris around buildings where larvae feed. Avoid fertilizing with excessive amounts of manure or similar organic materials.

Screen doors and windows and keep them closed to exclude flying insects. Plants with infested soil should not be brought indoors. Locate compost piles away from doors and windows and keep them covered. Aerate compost piles where fly larvae feed by periodic turning. Sterile potting soil should be stored in closed containers to prevent it from becoming infested.

Insecticides applied to potting soil can be effective for controlling larvae. Drenching soil with diazinon or carbaryl will kill larvae, but this can be hazardous. Only treat potted plants outdoors and don't bring them indoors until the insecticide treatment has dried or until the waiting period on the label has been exceeded. Strictly follow all directions and precautions on the pesticide label.

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